Jökull - 01.12.1983, Side 41

Tertiary (Miocene-Pliocene) interbasalt Sediments,
NW- and W-Iceland
ELEN ROALDSET
Norsk Hydro — Forskningssenteret, Lars Hilles gt. 30, 5000 Bergen, Norway.
ABSTRACT
The Tertiary NW- and W-parts of Iceland are mainly
built up of Jlood basalts, which in the west represent the
oldest exposed rocks in Iceland (14-16 m.y.) Interbedded
with the basalts extensive sediment horizons ocair, which
may refled longperiods of volcanic quietness. The sediments
consist of laterite, lignite, clay and tuffaceous material.
Their age can be determined indirectly by K-Ar dating and
paleomagnetic mapping of the lavas above and below.
The samples studied represent vertical sections through
selected interbasalt sediment sequences. The investigation
includes textural, mineralogical and chemical analyses of
sediments of different geological age (14-13, 10-9, 7-6.5,
and 4-3 m.y.).
Their composition is a result of various processes: pala-
gonitisation, weathering (kaolinite, halloysite, smectite,
amorphous Al-Fe-hydroxides), thermal metamorphism
(maghemite) and regional burial metamorphism/diagenesis
(heulandite, analcime). The oldest sediments have highest
kaolinite and halloysite contents. Theyounger are richer in
smectite, and in the youngest illite and vermiculite were
found.
INTRODUCTION
During the last 20-30 million years Iceland has
developed as a large basaltic plateau on the Green-
land — Faeroe Ridge. Tertiary plateau basalt. cov-
ers the main part of W and E Iceland (Fig. 1). The
lava pile of NW-Iceland consists of 4 1/2 km of
subaerial lavas of which 1 1/2 km is exposed and 2
km drilled. The lava pile has been modified by
isostatic sinking and glacial erosion. The flows have
variable thickness, 2-30 m, and are often intercalat-
ed with sedimentary beds from a few cm and up to
100 m in thickness (Dagley et al. 1967; Kristjánsson et
al. 1975).
By combining paleomagnetic and K/Ar data for
the lava succession with results from deep sea sedi-
ment cores stratigraphic boundaries are recorded:
The Miocene-Pliocene at 5.2 ± 0.1 m.y.
the Pliocene — Pleistocene boundary at 3.2 m.y.
(Berggren and van Couvering 1974, McDougall et al.
1976, 1977). The oldest glacial deposits in Iceland
have an age of about 3.1 m.y. (McDougall and Wen-
sink 1966).
The average building rates of the Tertiary lava
pile are 10-15 m per 10, 000 years (McDougall et al.
1976, 1977; Kristjánsson et al. 1975, Kristjánsson 1979,
pers. comm.) As the average flow thickness is 10-11
m this means that the mean time interval between
two successive flows is slightly higher than 10, 000
years. In comparison lavas from the flanks of
Hawaiian shield volcanoes show similar interflow
intervals, while intervals ofl 00-200 years have been
recorded within the summit calderas (Doell and Cox
1965).
The geology and geophysics of the Tertiary lavas
of the NW-peninsula, Vestfirðir, have been investi-
gated by Einarsson (1962), Kristjánsson (1968, 1973),
Haldet al. (1971), and Kristjánsson et al. (1975). The
oldest rocks in Iceland 14-16 m.y.occur here (Moor-
bath el al. 1969, K. Sæmundsson 1979, pers. comm.).
Minor zeolitization has taken place. The geology of
the younger Borgarfjörður area is reported by S<e-
mundsson andNoll (\91A),Jóhanneson (1972) and Mc-
Dougall et al. (1976). Chemically the lava suites plot
as quartz normative basalts following a tholeiitic
trend.
Interbasalt sediments are of common occurence
within the lava pile. The sediments consist of later-
itic soils, clays, lignite, hyaloclastite and tephra.
Sediment horizons of regional extension have been
traced (Kristjánsson et al. 1975, K. Sæmundsson 1979,
pers. comm., Fig. 1). The lateritic and lignite beds
commonly occur in the Miocene section but dis-
appear during the Pliocene probably in consistency
with the climatic change. The lignite occurrences
are comprehensively reviewed by Thoroddsen (1896)
and Friðriksdóttir (1978). In times of fuel shortage
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